1. Field of the Invention
The invention is in the field of monitoring the quantity of oil or liquid contained in a tank, and especially the troublesome area of measurement of the quantity of hot liquified asphalt--"asphalt oil"--as contained in and dispensed from tanks commonly of 100-ton capacity by asphalt contractors.
2. Prior Art
Asphalt paving is made up of a mixture of sand, crushed rock and asphalt. At ordinary temperatures asphalt is unmanageable because of its viscous, semi-solid, plastic character. Asphalt is therefore liquified by heating it to about 325.degree. F. for shipment in tank trucks and is maintained at that temperature in storage up until the time that it is formulated into the paving material. The liquified asphalt is popularly known as "asphalt oil," although that term may have another meaning in another context.
For many years, the practice among asphalt plant operators has been to estimate the quantity of asphalt oil in a tank by sending a workman to top of the tank with a flashlight and a "tankstick" or a tape measure, to throw open a heavy manhole cover and to peer into the dark, steamy, smelly tank while probing to determine the distance at which the end of the stick touches the oil, and to report that measurement to compute the quantity by reference to a chart.
Mechanical float devices which actuate a dial gauge have been noted in some older asphalt plants, but have fallen into disuse. One such gauge to be found at the Scott Contracting Company's plant in the Wisconsin Dells has three readings, FULL, HALF, and TURN OFF HEATER, which provide sparse information: the gauge has been stuck and useless for many years.
The use of a set of load cells under the supporting members of the tank has been considered but has been found to be cost prohibitive.
As a control systems troubleshooter and a supplier of controls to the asphalt plant industry, I was asked many times, particularly during the last two years, for better means to measure the quantity of oil. I sought long and diligently to locate a tank gauge to supply to my clients, but there were none to be found.
The Problem
An asphalt plant is a hazardous, highly mechanized and motorized environment involving materials handling on a grand scale. At the formulation area is found the operators' office and within it steel cabinets containing electrical controls to operate the conveying and mixing equipment. Huge tanks of up to 200-ton capacity store asphalt oil held at 325.degree. F. Huge rotating drum dryers work to heat and to drive off water from the sand and aggregate components. Metering pumps and conveyers work to carry the materials to rotating drum mixers or pug mills that blend the ingredients under the direction of the operator and a computer in the office, where the operator is confronted with a bank of dials, meters and controls. At the command of the operator, conveyers carry the hot asphalt mix high overhead to hoppers situated to dump the asphalt into trucks positioned below. Asphalt oil for the mixing operation is maintained in tanks near the site of the mixer. The tanks, typically are of about 100-ton capacity, being cylinders 12 feet in diameter and 30 feet in length. The focus of my invention is measurement of the quantity of asphalt oil in such a tank.
Tanksticking
The conventional way to measure the asphalt oil in the tank is to send a workman to the top of the tank with a "tankstick," which is a calibrated rod or a measuring tape. The workman must leave the security of the office, walk the distance from the office to the tank, climb up a 16 foot ladder, gain access to a manhole cover at the top surface of the tank, open a heavy manhole cover, insert a stick or a tape measure into the port, and observe when the end of the device reaches the top surface of the asphalt oil. This task is hazardous and disagreeable. The workman must virtually stick his head adjacent to the mouth of the tank to see anything within the tank because it is pitch-black inside. The workman may use a flashlight to assist him. The asphalt in the tank is held at 325.degree. F. by means of heating coils contained within the bottom of the tank which carry heat exchange liquid from a heater working outside the tank. The bituminous vapors arising from the opening of the manhole are noxious and are regarded as carcinogenic by industry critics. The vapors are flammable and will ignite upon exposure to a flame. There was a notorious accident in which a workman's flashlight failed while he was making a tankstick measurement and he set about to use his cigarette lighter to provide light. An explosion and fire resulted which seriously injured the workman and destroyed property.
When the workman has succeeded in making the measurement of the height of the empty space above the oil, the number is carried back to the office where it is used to figure the volume of oil in the tank by making reference to a chart.
The task of mounting the asphalt tank and making the tankstick measurement is an onerous and dangerous chore. It is important nonetheless that the plant operator be regularly apprised of the tank contents: (i) The asphalt tank must be regularly replenished from an on-site or tank-truck supply. The transfer is carried out by an operator who monitors the delivery using a metering pump and it is critical to have knowledge of how much oil can be delivered to the tank without overfill. An overfill results in the issue of hot asphalt oil from the top of the tank, the hot oil flowing out and washing over the same valve that must be used to shut off flow. The job of the workman who must turn off the valve is treacherous and can result in serious burns. (ii) If the tank contents is unknowingly depleted during the dispensing of oil from the tank during mixing operations, a batch of asphalt can be spoiled. (iii) If the contents of the tank is allowed to fall below a minimum level, there is danger that the oil will become overheated and catch fire unless steps are taken to moderate the flow of heat exchange fluid supplied by an external heater via coils within the bottom of the tank.
The situation that I report here has existed throughout my entire twenty-five year experience with asphalt plants. The idea that the problem could be avoided never occurred to anyone. The need for improved means for monitoring the contents of the asphalt oil tank has been unrecognized even though the need is glaringly implicit and long felt, but unmet over the long history of asphalt plant operations.
My invention addresses this long-felt need and solves the problem. My system continuously monitors the contents of the asphalt oil tank from inside the office and can be programmed to trigger an alarm if the asphalt oil exceeds a maximum fill level during fill operations or falls below the minimum level during dispensing operations. The continuous monitoring of the liquid level provides a cross check of the amount of asphalt oil delivered to any batch of asphalt mix. There is no need for the operator to exit the office, march to the base of the asphalt oil tank, climb the ladder, perch on top of the tank, remove the manhole cover, peer into the tank while endeavoring to see when the stick reaches the oil level, close the manhole cover, descend the ladder, march back to the office and figure the liquid level from the tankstick reading. My system simply reports the quantity on the face of an instrument mounted inside the office and in plain view of the operator. The peril to the tankstick operator can be ended. The operator's knowledge of the contents of the asphalt oil tank is continuous. Guesswork is ended.